1. Field of the Invention
This invention relates, in general, to electrical inductive apparatus and, more specifically, to electrical inductive apparatus having leakage flux shields.
2. Description of the Prior Art
In electrical inductive apparatus, such as transformers and the like, the flow of electrical current through the windings produces a magnetic leakage flux which passes through the magnetic core, tank walls and support structure causing overheating therein due to flux induced eddy and circulating currents. Various shielding means have been utilized to reduce the effect of the leakage flux and, thereby, limit overheating of the various components of the transformer. Magnetic shields have been employed on the tongue wedge and T-beam supports, as shown in U.S. Pat. Nos. 2,370,045 and 3,281,745, both assigned to the assignee of the present invention, to conduct the leakage flux along the shield rather than into the support structure. Bundles of magnetic material have been employed, as in U.S. Pat. No. 3,464,041 issued to Waterman, between the windings and core legs to carry the leakage flux to a secondary magnetic core which provides a return path for the flux thereby preventing it from entering the laminations of the core and generating eddy currents therein. Magnetic shields have also been used in U.S. Pat. No. 3,821,677, issued to Boaz and assigned to the assignee of the present invention, to divert the leakage flux from the tank surfaces to a low reluctance path consisting of laminations of ferrite magnetic material which form closed magnetic paths around the windings.
Leakage flux has also been controlled by means of an electrically conductive shield disposed between the turns of the windings, as in U.S. Pat. No. 3,142,029, and between the structural members supporting the transformer in a tank, as in U.S. Pat. No. 3,827,018, issued to Thomas and assigned to the assignee of the present invention. The electrically conductive shields create a counter magnetic field which opposes the leakage flux and thereby reduces the total amount of the leakage flux passing through the shields to the tank and other support structure.
The aforementioned magnetic and electrically conductive shields effectively control the leakage flux in their vicinity. However, stray components of the leakage flux pass through various parts of the transformer not protected by these shields and cause undesirable heating therein. One component or portion of the leakage flux that has not been effectively controlled by the use of prior art flux shields is that component which passes or cuts through the core window. In an electrical transformer, such as a shell-form type, the magnetic core forms a loop around the windings. A voltage is induced around this loop by the winding leakage flux which cuts through the core window. Although the individual core laminations are coated with an insulating material, electrical contact is still made between the laminations through burrs produced in the manufacture of the laminations. These burrs or points of contact form a path for current, forced by the induced voltage, to flow in the magnetic core. When flowing through these points of contact, the current causes hot spots and severe burning of the core laminations which, in turn, produces combustible gasses.
Thus, it would be desirable to provide an electrical inductive apparatus, such as a transformer, having flux shields which prevent the winding leakage flux from entering the core laminations and causing hot spots and the production of combustible gas therein. It would also be desirable to provide an electrical inductive apparatus having improved flux shields which can be installed without substantially increasing the dimensions of the magnetic core and coil assembly of the apparatus.